Numerous techniques have been developed or advanced for controlling the volume of air or other gas to be delivered to a patient suffering from one or more respiratory deficiencies. Particularly in conjunction with portable ventilators, it is known to utilize complicated ratchet and gear mechanisms together with a brush-type electric motor for controlling the movement of a piston. The retracted position of the piston, at one end of its cycle, defines the volume of air to be provided to the user of the ventilating machine. It is common practice for the operator to be able to adjust the volume of air or gas to be delivered to a patient using such machines. Such adjustment is important so that individualized respiratory treatment is available, i.e. the volume of air and the velocity of piston movement can be controlled to achieve a desired inspiration/expiration cycle for each patient. In U.S. Pat. No. 4,141,355 to Apple, issued Feb. 27, 1979 and entitled "Apparatus For Automatic Ventilation of the Lungs", a mechanism is disclosed for changing the volume of air that can be delivered and is characterized by the use of a ratchet wheel, a sliding block, and a jack screw. These members cooperate to effect movement of a piston in order to change the volume of air that is being provided to a patient.
Although prior art devices may work satisfactorily, there remains a need for a ventilator that reduces the number of mechanical components used to control the amount of gas for ventilating a patient and thereby achieve lower mechanical inertia in the total respiratory system.